1. Field of the Invention
This invention pertains to treating wells to increase production rate by hydraulic fracturing. More particularly, methods are provided for increasing flow rate of wells by injecting a highly viscous material, which may be a thermoplastic degradable polymer, which may contain proppant particles, into an earth formation surrounding a well. In other embodiments, methods are provided to improve conventional hydraulic fracturing and sand control processes.
2. Discussion of Related Art
Hydraulic fracturing of earth formations around a wellbore for increasing fluid productivity or injectivity of the well is a mature technology. Normally, thousands of gallons of an oil-based or water-based fluid, usually made viscous by addition of a soluble polymer, are injected into a formation at an injection rate such that the pressure of the fluid at the formation is higher than the earth stress in the formation. This causes a crack or fracture to develop in the face of the rock at the wellbore. Continued fluid injection into the well then causes the fracture to increase in length and width. After a sufficient width is achieved by continued fluid injection, particles, called “proppant,” are added to the fluid. The fluid injected up until the time proppant is added is called “pad” fluid. After fluid injection has ceased, fracturing fluid flows out of the fracture, allowing the walls of the fracture to close on the proppant. The proppant particles then “prop” the walls of the fracture apart. Since proppant particles are normally much larger than the particles of the formation, the fluid permeability of the propped fracture is much greater than that of the formation; hence, the flow capacity of the well is increased. Fractures are often propped out to distances from about 200 feet to thousands of feet in each of two opposite directions from the wellbore in low permeability formations. Fractures as short as 25 feet may be formed in high permeability formations using convention gelled fluids and as short as 5 feet in high permeability formations using water as a fracturing fluid (“Water-Fracturing vs. Frac-Packing: Well Performance Comparison and Completion Type Selection Criteria,” SPE 38593, Society of Petroleum Engineers, 1997). The flow rate into a well in a low-permeability formation can be increased several-fold by the hydraulic fracture, depending on the properties of the formation, the proppant and the geometry of the propped fracture. Along with the thousands of gallons of fracturing fluid, thousands of pounds of proppant are normally injected in a fracturing treatment in a low permeability formation, although it has been reported that in some wells injection of fluid alone (i.e., without proppant) at fracturing pressures has increased production rate. At the end of a fracturing treatment, proppant-laden fluid is “flushed” from the wellbore into the formation by a proppant-free displacement fluid, which is usually brine.
In more recent years, application of hydraulic fracturing along with gravel packing of wells has become very common, in a process often called “frac-packing.” The formations in which this process is usually applied have high permeability, and the fracture is formed only to bypass permeability damage near a wellbore (SPE 38593, referenced above). It was recognized that a short fracture, in the range of 2 to 8 feet in length, could bypass damage in Gulf of Mexico wells that require sand control (SPE 38593, p. 286). Some evidence exists that conventional fracturing fluids, containing a soluble gelling polymer, damaged the permeability of the longer fractures formed with conventional fluids when gravel packing, whereas much shorter fractures formed with water appeared to be just as effective.
For hydraulic fractures formed by conventional processes to be effective, the fracture must be propped all the way to the wellbore. Even a short distance of unpropped fracture or of proppant with damaged permeability can greatly diminish or even eliminate the benefits of the long propped fracture. Therefore, there is need for materials and method to connect long propped hydraulic fractures in wells that have already been fractured all the way to the wellbore and eliminate any lack of proppant or damage to the propped fracture within a relatively short distance from a wellbore.
There is also a need for a method to prevent “overflushing” of proppant away from the wellbore by the displacement fluid at the end of a conventional fracturing treatment. Overflushing can be responsible for the lack of proppant in the fracture very near the wellbore.
In some hydraulic fracturing treatments, “flowback” of proppant is observed when the well is produced. This is a need for method to prevent this flow of proppant out of the fracture and into the wellbore after a fracturing treatment.
One of the benefits from hydraulic fracturing of many wells is removal of “damage” to flow capacity near a well. Damage removal in connection with gravel packing of very high permeability formations was discussed above, but damage to flow capacity of wells in all permeability ranges is widely observed. The mechanisms causing damage have been studied extensively, and include: clay blockage from drilling fluids, damage to rock permeability from shooting perforations into the formation, perforation plugging or formation damage from leakoff of completion fluids, migration of fine particles from the formation to the near-well region, and deposition of chemical scales from produced or injected water, or combinations of these phenomena. The distance to which damage extends from a wellbore is not known in each well, but it is generally believed to be not more than a few feet. Well stimulation methods that are generally used for damage removal near a wellbore include limestone and sandstone acidizing and solvent injection, but often these treatments are not successful or their effectiveness quickly diminishes as fluid is produced from a well. A hydraulic fracturing method and materials are needed to form a propped fracture having a high fluid flow capacity and extending only through the damage zone near a wellbore, or a relatively short distance from a wellbore, without the requirements of pumping large amounts of fluid and proppant into the well. The method should be applicable to a wide range of permeabilities of the formation around the well. This method can be especially needed in remote areas, where mobilizing of materials and equipment is expensive. The method is also needed to remove damage before gravel packing a well.
Theoretical models to predict the geometry of hydraulic fractures formed around wells have been developed. The models generally indicate that the width of a fracture at the wellbore increases with effective viscosity of the injected fluid in the fracture, rate of fluid injection and volume of fluid injected. To achieve a fracture width that can accept proppant without using large quantities of fluid, effective viscosity of the fracturing fluid must be high. But, viscosity of a fracturing fluid is normally limited by pressure loss as the fluid is pumped down a wellbore. Presently-used fracturing fluids minimize this pressure loss by employing polymer solutions that are highly non-Newtonian (shear-thinning). Otherwise, pressure loss due to friction in the tubing would allow injection only at very small rates. Water-soluble polymers are cross-linked to increase viscosity, and this cross-linking is sometimes delayed to decrease pressure loss in tubulars. Another limit on increasing viscosity of present fracturing fluids is that the water-soluble polymers most commonly used do not completely degrade, but leave a residue that adversely affects flow capacity of the proppant left in a fracture. Higher polymer concentrations in the injected fluid to produce higher viscosity and wider fractures would cause even greater damage to proppant flow capacity in the fracture. Other viscous solutions, based on surfactant molecules, have been developed, but they have disadvantages such as the uncertainty of positive reduction of viscosity with time and high cost. All presently used fracturing fluids have the disadvantage that large quantities of liquid must be pumped into a well because the effective viscosity of the fluid in a fracture is limited.
A process using high effective viscosity fracturing fluid for damage removal must provide a method for controlling pressure loss in tubulars when the fluid is injected into a well. A process that allowed pumping high-viscosity oil at a high rate with low friction loss in tubulars by using a “water ring” was developed (the “Superfrac process”, “A New Hydraulic Fracturing Process,” J. Pet. Tech., January 1970, 89-96). Difficulties in handling the oil and in controlling the process caused the process to lose favor, but the concept of using a lubricating layer in tubing to inject a viscous fracturing fluid was demonstrated. Large quantities of fracturing fluid were employed in this process, also. Since a water ring was used, the viscous oil was not the external phase in the tubing; this made possible low friction loss in tubing. If water was the external phase in a hydraulic fracture, viscous oil did not have high effective viscosity in the fracture, either. The effective viscosity of a viscous oil fracturing fluid flowing in the fracture would have been much lower than the viscosity of the oil, although the high viscosity of the oil was effective for proppant transport. The effective viscosity for flow in a fracture when using the “Superfrac” process is not known.
If a fracturing fluid has high viscosity, the fluid must degrade such that flow can be established through the fracture after the treatment. Therefore, material and method are needed for a fracturing fluid that can be placed in the wellbore near the zone to be fractured without excessive pressure loss in the wellbore, that can provide high effective viscosity in the fracture, and that can provide a method for the fluid to degrade to allow flow of well fluids through the fracture.